Process for producing a planar body of an oxide single crystal

ABSTRACT

A planar body with a good crystallinity is grown continuously and stably when a planar body of an oxide single crystal is grown by a micro pulling-down method. A raw material of the oxide single crystal is melted in a crucible  7 . A fibrous seed crystal  15  is contacted to a melt  18 , and then the melt  18  is pulled down from an opening  13   c  of the crucible  7  by lowering the seed crystal. A shoulder portion  14 A is produced following the seed crystal, and a planar body  14 B is produced following the shoulder portion. In this case, differences in lattice constants between each crystal axis of the seed crystal and each corresponding crystal axis of the shoulder portion are controlled at 1% or less, respectively.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a process for producing a planar body ofan oxide single crystal.

[0003] 2. Description of the Related Art

[0004] A single crystal of lithium potassium niobate and a singlecrystal of lithium potassium niobate-lithium potassium tantalate solidsolution have been noted especially as single crystals for a blue lightsecond harmonic generation (SHG) device for a semiconductor laser. Thedevice can emit even the ultraviolet lights having the wavelengths of390 nm or so, thus the crystals can be suitable for wide applicationssuch as optical disk memory, medicine and photochemical fields, andvarious optical measurements by using such short-wavelength lights.Since the above single crystals have a large electro-optic effect, theycan be also applied to optical memory devices using theirphoto-refractive effect.

[0005] However, for an application of a second harmonic generationdevice, for example, even a small fluctuation in a composition of thesingle crystal may affect the wavelength of the second harmonic wavegenerated by the device. Therefore, the specification of the range ofthe composition required for said single crystals is severe, and thefluctuation in the composition should be suppressed in a narrow range.However, since the composition consists of as many as three of fourcomponents, growing a single crystal at a high rate is generallyextremely difficult to achieve, while controlling the proportion of thecomponents to be constant.

[0006] In addition, for optical applications, especially for anapplication for the second harmonic wave generation, a laser beam havinga short wavelength of, for example, about 400 nm needs to propagate inthe single crystal at as a high power density as possible. Moreover, thephoto deterioration has to be controlled to the minimum at the sametime. In this way, since controlling the photo deterioration isessential, the single crystal has to possess a good crystallinity forthis purpose.

[0007] Moreover, lithium niobate and lithium potassium niobate can besubstituted with cations, thus solid solution in which the cations aresolid-solved is produced. Therefore, controlling the composition of themelt needs to grow a single crystal of a specific composition. From sucha background, a double crucible method and a method of growing a crystalwhile feeding raw materials are examined mainly for the CZ method andthe TSSG method. For example, Kitamura et al. tried to grow a lithiumniobate single crystal of a stoichiometric composition by combining anautomatic powder feeder to a double crucible CZ method (J. CrystalGrowth, 116 (1992), p.327). However, it was difficult to enhance acrystal growth rate with these methods.

[0008] NGK Insulators, Ltd. suggested a micro pulling-down method forgrowing the above single crystal with a constant compositionalproportions, for example, in JP-A-8-319191. In this method, a rawmaterial, for example, comprising lithium potassium niobate is put intoa platinum crucible and melted, and then the melt is pulled downgradually and continuously through a nozzle attached to the bottom ofthe crucible. The micro pulling-down method can grow a single crystalmore rapidly than the CZ method or the TSSG method does. Moreover, thecompositions of the melt and the grown single crystal can be controlledby growing the single crystal continuously with feeding the rawmaterials for growing the single crystal to the raw material meltingcrucible.

[0009] However, there is still a limitation in using a micropulling-down method to grow a good single crystal plate (a planar bodyof a single crystal) continuously at a high rate. Because, when theplanar body of the single crystal is pulled down with a planar seedcrystal, cracks tend to occur near an interface boundary between theseed crystal and the planar body.

SUMMARY OF THE INVENTION

[0010] The inventors found that cracks were likely to occur as thedifference in lattice constant between the seed crystal and the planarbody became greater, and suggested a method for preventing cracks bymatching the lattice constant of the seed crystal with that of theplanar body at a high accuracy in Japanese Patent application No.2000-065123 (filed on Mar. 9, 2000). However, it was practically verydifficult for a multi-component solid solution system as in lithiumpotassium niobate-lithium potassium tantalate solid solution to matchthe lattice constant of a seed crystal with that of a planar body at ahigh accuracy.

[0011] It is an object of the invention to prevent cracks from occurringnear the interface boundary between a seed crystal and a planar body,and to grow a planar body of an oxide single crystal having a goodcrystallinity continuously and stably with a simple technique, when theplanar body of the oxide single crystal is grown with the micropulling-down method.

[0012] The inventors had examined various methods to grow planar bodiesof oxide single crystals, which used the micro pulling-down method. As aresult, the inventors found that cracks were more unlikely to occur, asan area of an interface boundary between the seed crystal and the planarbody became smaller. Thus, a planar body having a good crystallinity wascontinuously made by melting a raw material of an oxide single crystalin a crucible, contacting a fibrous seed crystal to a melt, pulling downthe melt from the opening of a crucible by lowering the seed crystal,forming a shoulder portion following the seed crystal, forming a planarbody following the shoulder portion, and controlling differences inlattice constants between each crystal axis of the seed crystal and eachcorresponding crystal axis of the shoulder portion at 1% or less (morepreferably 0.5% or less), respectively.

[0013] In this case, the lattice constant of each crystal axis of theshoulder portion can be adjusted by controlling the proportions ofrespective components in the crucible. Taking lithium potassium niobate,for example, the lattice constants of each crystal axis in the grownplanar body can be changed by slightly changing a relative ratio ofniobium, lithium and potassium in the crucible.

[0014]FIG. 1 is a schematic sectional view of a manufacturing apparatusfor growing a single crystal.

[0015] FIGS. 2(a) through (c) represent steps of pulling down a planarbody of the single crystal.

[0016] A crucible 7 is placed in a furnace body. An upper furnace unitis arranged to surround the crucible 7 and an upper space 5 thereof, andhas a heater 2 buried therein. A nozzle 13 extends downwardly from abottom part of the crucible 7. The nozzle 13 comprises a connecting-tubeportion 13 a and a planar expanded portion 13 b at the lower end of theconnecting-tube portion 13 a. In FIG. 1, only a cross sectional view ofthe planar expanded portion 13 b is shown. The connecting-tube portion13 a and the planar expanded portion 13 b can be changed variously inshape. Both 13 a and 13 b can also be arbitrarily changed incombination. A slender opening 13 c is formed at the lower end of theplanar expanded portion 13 b, and a vicinity of the opening 13 c is asingle crystal-growing portion 35. A lower furnace unit 3 is arranged tosurround the nozzle 13 and a surrounding space 6 thereof, and has aheater 4 buried therein. The crucible 7 and the nozzle 13 are bothformed from a corrosion-resistant conductive material.

[0017] One electrode of a power source 10 is connected to a point A ofthe crucible 7 with an electric cable 9, and the other electrode of thepower source 10 is connected to a lower bent B of the crucible 7. Oneelectrode of an another power source 10 is connected to a point C of theconnecting-tube portion 13 a with an electric cable 9, and the otherelectrode of the power source 10 is connected to a lower end D of theplanar expanded portion 13 b. These current-carrying systems areisolated from each other and configured to control their voltagesindependently.

[0018] An after-heater 12 is located in the space 6 to surround thenozzle 13 with a distance. An intake tube 11 extends upwardly in thecrucible 7 and an intake opening 22 is provided at the upper end of theintake tube 11. The intake opening 22 slightly protrudes from a bottomportion of a melt.

[0019] The upper furnace unit 1, the lower furnace unit 3 and theafter-heater 12 are allowed to heat for setting an appropriatetemperature distribution in each of the space 5 and space 6. Then a rawmaterial for the melt is supplied into the crucible 7 and theelectricity is supplied to the crucible 7 and the nozzle 13 for heating.In this condition, the melt slightly protrudes from the opening 13 c atthe single crystal-growing portion 35.

[0020] In this condition, a fibrous seed crystal 15 is moved upwardly asshown in FIG. 2(a), and the upper surface of the seed crystal 15 iscontacted with the melt protruding from the opening 13 c. At that time,a uniform solid phase-liquid phase interface (meniscus) is formedbetween the upper end of the seed crystal 15 and the melt 18 pulleddownwardly from the nozzle 13. Then, the seed crystal 15 is lowered asshown in FIG. 2(b). As a result, a shoulder portion 14A is continuouslyformed on an upper side of the seed crystal 15 and pulled downwardly.

[0021] The width of the shoulder portion 14A gradually increases fromthe seed crystal to the crucible. The angle of the shoulder portiondepends on the type and the composition of the crystal, a temperature ofthe single crystal-growing portion 35, the pulling-down rate, and thelike. When the temperature of the single crystal-growing portion 35 israised a little, the shoulder portion 14A stop increasing its width, andthe planar body 14B with a constant width is continuously grown afterthat.

[0022] A smaller contact area between the melt and the seed crystal ispreferable from the viewpoint of preventing cracks. However, the seedcrystal may break due to a lack of strength when the seed crystal is toothin. Therefore, regardless of the width of the planar body, the seedcrystal preferably has the width of 1-3 mm.

[0023] The lattice constant is measured by an X-ray diffractionapparatus (MRD diffractometer, manufactured by Philips). If anunequivalent crystal axis exists in an oxide single crystal, differencesin lattice constants between each crystal axis of the seed crystal andeach corresponding crystal axis of the shoulder portion, respectively,have to be 1% or less.

[0024] An oxide single crystal is not particularly limited, but, forexample, lithium potassium niobate (KLN), lithium potassiumniobate-lithium potassium tantalate solid solution (KLTN:[K₃Li_(2-x)(Ta_(y)Nb_(1-y))_(5+x)O_(15+2x)]), lithium niobate, lithiumtantalate, lithium niobate-lithium tantalate solid solution,Ba_(1-x)Sr_(x)Nb₂O₆, Mn—Zn ferrite, yttrium aluminum garnet substitutedwith Nd, Er and/or Yb, YAG, and YVO₄ substituted with Nd, Er, and/or Ybcan be exemplified.

EXAMPLE 1

[0025] With a single crystal-producing apparatus shown in FIG. 1, aplanar body of a lithium potassium niobate single crystal was producedaccording to the invention. Specifically, the temperature of the wholefurnace was controlled by the upper furnace unit 1 and the lower furnaceunit 3. The apparatus was configured to be able to control thetemperature gradient near the single crystal-growing portion 35 by anelectric supply to the nozzle 13 and the heat generation of theafter-heater 12. A mechanism of pulling down the single crystal platewas equipped, in which a single crystal plate was pulled down withcontrolling the pulling-down rate uniformly within a range from 2 to 100mm/hour in a vertical direction.

[0026] A fibrous seed crystal of lithium potassium niobate was used. Asize of the seed crystal was 1 mm×1 mm in cross-section and 15 mm inlength. Lattice constants of the seed crystal were 12.60 Å in a-axis and3.97 Å in c-axis. The molar ratio of potassium, lithium and niobium was30:14:56. A half width of an X-ray rocking curve was 80 seconds at 0 0 4reflection of the seed crystal (measured by the MRD diffractometer,manufactured by Philips). The seed crystal was held at a holding rodwith a heat-resistance inorganic adhesive, and the holding rod wasconnected to the pulling-down mechanism.

[0027] Potassium carbonate, lithium carbonate and niobium pentoxide wereprepared at a molar ratio of 30:25:45 to produce a raw material powder.The raw material was supplied into the platinum crucible 7, and thecrucible 7 was set in place. With controlling the temperature of thespace 5 in the upper furnace unit 1 within a range from 1100 to 1200°C., the raw material in the crucible 7 was melted. The temperature ofthe space 6 in the lower furnace unit 3 was controlled uniformly withina range from 500 to 1000° C. While a given electric power was suppliedto each of the crucible 7, the nozzle 13 and the after-heater 12, asingle crystal was grown. In this case, the temperature of the singlecrystal-growing portion could be at 980-1150° C., and the temperaturegradient of the single crystal-growing portion could be controlled at10-150° C./mm.

[0028] The crucible 7 had an elliptical cross-sectional shape, whereinthe major axis, the minor axis and the height was 50 mm, 10 mm and 10mm, respectively. The length of the connecting-tube portion was 5 mm. Across-sectional dimension of the planar expanded portion 13 b was 1mm×50 mm. A dimension of the opening 13 c was 1 mm long×50 mm wide.Under such conditions, the seed crystal 15 was pulled down at a rate of10 mm/hour.

[0029] As a result, a solid phase-liquid phase interface was descendedand a lower portion of a melt band was cooled to the temperature belowits crystallizing point to gradually crystallize a single crystal at anupper end of the seed crystal. When the seed crystal was furtherlowered, single-crystallization was continuously progressed to form ashoulder portion 14A. When the temperature of the single crystal-growingportion was lowered by changing temperatures of the crucible 7, thenozzle 13 and/or the after-heater 12, single-crystallization wasenhanced to increase a spreading angle θ of the shoulder portion 14A. Incase of the spreading angle θ being too large, the spreading angle θ wascontrolled by raising the temperature of the single crystal-growingportion 35. The spreading angle θ was kept constant at about 30 degrees.While controlling the spreading angle θ, the shoulder portion 14A waskept growing. An area of the shoulder portion 14A gradually increased tofinally reach 35 mm wide and 1 mm thick. At this time, the width of theplanar body 14B was controlled at 35 mm uniformly by suppressing thecrystallization through raising the temperature of the singlecrystal-growing portion 35.

[0030] While the raw material in equal weight to that of thecrystallized melt was being fed to the crucible 7, the crystal was keptgrowing, and the planar body was cut off from the nozzle 13 and cooledwhen the total length of the shoulder portion 14A and the planar body14B reached 100 mm. The lattice constant of the shoulder portion of theobtained planar body was measured to give the a-axis length of 12.58 Åand the c-axis length of 4.01 Å. A molar ratio of potassium, lithium andniobium was 30:17:53, respectively. A difference between the latticeconstant of the shoulder portion and that of the seed crystal (latticemismatch) was 0.15% in a-axis and 1.0% in c-axis. However, no crackoccurred at the joining portion between the seed crystal and theshoulder portion. Also, a half width of an X-ray rocking curve was 50seconds at 0 0 4 reflection of the shoulder portion.

EXAMPLE 2

[0031] Similar results as Example 1 were obtained with a plate oflithium potassium niobate-lithium potassium tantalate solid solutionsingle crystal.

EXAMPLE 3

[0032] A planar body of lithium niobate was grown according to Example 1except that a fibrous seed crystal of lithium niobate was used.Dimensions of the seed crystal were 1 mm×1 mm in cross-section and 15 mmin length. The seed crystal was cut out and obtained from astoichiometric lithium niobate single crystal grown by the Czochralskimethod. A direction of pulling down the seed crystal was set parallel tothe X-axis, and a direction of a growing face was set parallel to theZ-axis. A lattice constant of the seed crystal was 5.150 Å in a-axis and13.864 Å in c-axis. A molar ratio of lithium and niobium was 48.6:51.4,respectively. A half width of an X-ray rocking curve was 12 seconds at 00 12 reflection of the seed crystal.

[0033] Lithium carbonate and niobium pentoxide were prepared at themolar ratio of 58:42 to produce a raw material powder. The raw materialwas fed into the platinum crucible 7, and the crucible 7 was arranged ina predetermined place. While a temperature of the space 5 in the upperfurnace unit 1 was controlled within a range from 1200 to 1300° C., theraw material in the crucible 7 was melted. A temperature of the space 6in the lower furnace unit 3 was controlled uniformly within a range from500 to 1000° C. While a predetermined electric power was supplied toeach of the crucible 7, the nozzle 13 and the after-heater 12, a singlecrystal was grown. In this case, the temperature of the singlecrystal-growing portion could be at 1200-1250° C., and the temperaturegradient of the single crystal growing portion could be controlled at10-150° C./mm. The seed crystal was lowered at a rate of 30 mm/hour. Avolume of the crystallized lithium niobate was measured to convert it toa weight every unit time internally, and the raw material of lithiumniobate in equal weight to the converted one was supplied into thecrucible. In this case, unlike the raw material powder melted at thebeginning, the lithium niobate powder supplied afterward in such mannerwas prepared to have a molar ratio of lithium and niobium at 50:50. Whenthe width of the shoulder portion 14A reached 50 mm, a planar body 14Bhaving the width of 50 mm was grown by controlling the temperature ofthe nozzle 13.

[0034] While the raw material powder of lithium niobate was being fed,the crystal was continuously grown until the total length of theshoulder portion 14A and the planar body 14B reached 100 mm. Then theplanar body was cut off from the nozzle 13 and cooled.

[0035] The composition of the shoulder portion of the obtained planarbody was measured by an inductively coupled plasma method, and a molarratio of lithium and niobium was 50:50, which corresponded to thestoichiometry composition. A lattice constant of the planar body wasmeasured to give an a-axis length of 5.148 Å and the c-axis length of13.857 Å. Differences in lattice constant between the shoulder portionand the seed crystal (lattice mismatch) were 0.04% in a-axis and 0.05%in c-axis. No cracks occurred at the joining portion between the seedcrystal and the shoulder portion. A half width of an X-ray rocking curvewas 12 seconds at the shoulder portion.

EXAMPLE 4

[0036] A planar body was grown according to Example 3 except that theseed crystal was worked so that a direction of pulling down the seedcrystal might be parallel to the Z-axis, and that a direction of growingface might be parallel to the X-axis. An a-axis length, a c-axis lengthand a half width of X-ray rocking curve at the seed crystal and theshoulder portion were similar to those in Example 3. No crack alsooccurred at the joining face between the shoulder portion and the seedcrystal.

[0037] As mentioned above, according to the invention, when the planarbody of the oxide single crystal was grown by the micro pulling-downmethod, cracks can be prevented near an interface between the seedcrystal and the planar body, and the planar body with good crystallinitycan be grown continuously and stably.

What is claimed is:
 1. A process for producing a planar body of an oxidesingle crystal, said process comprising the steps of melting a rawmaterial of said oxide single crystal in a crucible, contacting afibrous seed crystal to the melt, pulling down said melt from an openingof said crucible by lowering the seed crystal, forming a shoulderportion following said seed crystal, and producing said planar bodyfollowing the shoulder portion, wherein differences in lattice constantsbetween each crystal axis of said seed crystal and each correspondingcrystal axis of said shoulder portion are 1% or less, respectively.
 2. Aprocess for producing a planar body according to claim 1 , wherein saiddifferences in lattice constants between each crystal axis of said seedcrystal and each corresponding crystal axis of said shoulder portion areadjusted by controlling a proportion of each component in the crucible.3. A process for producing a planar body according to claim 1 , whereinsaid shoulder portion stop increasing its width and then said planarbody with constant width is continuously pulled out by raising thetemperature of a single crystal-growing portion.
 4. A process forproducing a planar body according to claim 1 , wherein a spreading angleof said shoulder portion is controlled by controlling the temperature ofa single crystal-growing portion.